Under which of these conditions will no image be formed (real or virtual) for an object placed in front of a concave mirror?

At the focus of the mirror. There will be no image formed. So the type of this when it is in the focus of the mirror then it will have no image form. When it is at the curvature it will form a real image as well as at the center of the focus and curvature. The next position to it will give you a virtual image.

Arise 8 run 1 this means go 8 up and 1 across, which is steepest